1. Field of the Invention
The present invention relates generally to the metal casting art and is more particularly concerned with novel components made by low pressure/high velocity plasma spray-casting which have unique microstructures and, consequently, novel combinations of physical properties superior to the physical properties of components of the same alloy compositions made by other methods. Articles of this invention having special utility because of the aforementioned superior physical properties, such as elevated temperature strength and ductility and thermal fatigue resistance, include, for example, gas turbine engine airfoils and discs made from nickel-base, cobalt-base or iron-base superalloys. Also included are articles, such as flywheels, which operate in lower temperature regimes than gas turbine engine parts, but present special physical property requirements problems and articles not readily obtainable by conventional methods due to their geometries or material compositions or combinations of the two.
2. Description of the Prior Art
Since the advent several decades ago of the gas turbine in various forms as a power generating and propulsion machine of major importance, there has been general recognition of the limitations imposed on the operation and performance of these machines by the available materials of construction. Relatively high tensile strengths and good ductility from room temperature to gas turbine combustion chamber operating temperatures, and good resistance to thermal fatigue, are highly desirable in combination in such applications. Extensive research and development efforts continuing to the present time have resulted in today's so-called "superalloys", prominent among which are the nickel-base, cobalt-base, iron-base and chromium-base refractory alloys including, for example, those marketed under the designation Rene' 80, Rene' 95, IN 738, IN 617 and IN 671.
Because of the importance of the strength requirements, especially in the lower temperature ranges of gas turbine operation, the first four of these alloys are used in airfoil and disc fabrication, whereas IN 671 is used primarily for its environmental resistance. Usually this use involves IN 671 in wrought sheet form, but it has been proposed as a plasma-sprayed coating applied directly on the article to be protected. The other four alloys are, however, generally melted and cast to shape and size or mechanically deformed from a cast or powder metallurgical body for ultimate use as airfoils and other gas turbine hot section components. Regardless of the fabrication method though, parts made of these alloys may require corrosion protection which presently takes the form in many cases of a spray-on coating of one of the MCrAlY alloys.
While, as indicated above, considerable advances in materials development have been made in answer to the special requirements of gas turbine engines, important materials capabilities are still lacking. To this time, however, superalloys used in fabricating gas turbine hot stage components have represented compromises between the above-mentioned various physical properties, operating conditions and manufacturing operations. This is particularly the case with those parts which are cast directly to shape. Thus, no new supperalloy which eliminates the necessity for such compromises in the production of cast components for gas turbine engines has materialized and no other alternative route to the elimination of those compromises has appeared prior to the present invention.
Another approach represented by the process of U.S. Pat. No. 4,066,117, issued Jan. 3, 1978, to Clark et al, entitled "Spray Casting of Gas Atomized Molten Metal to Produce High Density Ingots", involves as an essential step the mechanical deformation of the cast body as by forging to convert the superalloy ingot to an airfoil or other engine component form.